Implantable pulse generators are now being used for cardiac pacemakers, defibrillator, cardioverters, neuro stimulators including those with leads implanted into the brain for controlling tremor, those with leads implanted into the spine for controlling continuous pain, and so forth. A problem common to all such devices includes the determination of the ability of the lead to transmit energy and a way to provide a reliable measurement of this lead capability.
In particular, implantable pulse generators used for pacing a patient's heart, pacemakers, may perform a critical function without which the patient may die nearly immediately, that is it may provide the stimulus required to keep the heart beating in cases of heart block and in cases where the patient has obtained a transplanted heart, for example.
If a lead essential to pacing the heart were to fail, automatic response to such failure may mean the difference between the life and the death of the patient. Accordingly, for many years since the start of cardiac pacing, the issue of the integrity of the conductors for conducting electrical stimulating pulses to pace the heart from the implanted pulse generator to the site of connection to the heart has been a serious concern and many solutions have been proposed to both provide for automatic responsiveness by shifting the pacing pulse from a bad conductor to an alternative good conductor and to creating at least a minimal historical record of the measurements of the pacing leads conductors impedance so as to generate data which can be used to redesign a next generation of leads or possibly to warn of a impending lead conductor failure.
The reason this problem is particularly acute in heart pacing is because lead conductors are usually metal which flexes constantly under the repeated motions of the heart causing metal fatigue, pacing leads are also susceptible to the possibility of insulation failure which would expose the metal conductors to the environment of the body which is particularly hostile to maintaining small metal wires or coils of wires in optimum condition.
In U.S. Pat. No. 5,003,975 issued to Hafelfinger et al, a good description of prior art solutions maybe found. It describes U.S. Pat. No. 4,140,131 (Dutcher et al.), U.S. Pat. No. 4,549,548 (Wittkampf et al); U.S. Pat. No. 4,606,349 (Livingston et al.); and these patents are hereby incorporated by this reference hereto in their entireties. Additional patents by Walhstrand et al, U.S. Pat. No. 5,534,018; Kuehn, U.S. Pat. No. 5,201,865, Steinhaus et al, U.S. Pat. No. 5,201,808 Hudrlik, U.S. Pat. No. 5,156,149, Wayne et al, U.S. Pat. No. 5,137,021 Ekwall, U.S. Pat. No. 4,899,750; Collins, U.S. Pat. No. 5,184,614; and et al, U.S. Pat. No. 5,350,410; and Hansen et al, U.S. Pat. No. 5,431,692; also describe method and apparatus for sensing and using lead impedance for determining the integrity and or connection of lead conductors to the heart. Accordingly, these patents are also incorporated by this reference hereto in their entireties. Most of these patents just above listed depend on the generation of an impedance reading during a period of time when the pacemaker is not providing a stimulation pulse to the heart or alternatively they sample and hold some portion or portions of a pacing signal, digitize some characteristic or characteristics inherent in that signal and have that digitized signal representation considered by program run by a microprocessor in order to produce a signal value or a number to indicating a good or bad value for the conductor under test.
One thing the art has not yet shown is a practical system through which the pacing pulse may be used to derive an impedance measurement based integrity value nearly contemporaneously with the pacing pulse and without requiring significant microprocessor involvement or power usage. Ideally such a system would be able to distinguish between short or open circuits in the pacing path (or other stimulator pathway) and enable the implantable pulse generator to switch to alternative pathways within a single cardiac cycle. We teach this in another application filed on even date herewith and entitled Pacing Lead Current Monitoring Circuit, hereby incorporated by this reference thereto in its entirety.
Another failing in the prior art is its inability to distinguish non-physiologic senses(called herein, "NPS" events, which are rapidly occurring events sensed by the sense amplifier but which have no physiologic origin) from a physiologic sensed event. Progress has been made towards this by Ekwall in the '750 patent cited above, but it teaches only how to find the peaks which may also be noise coupling. We have noticed that the falloff from a peak caused by a broken lead will be slower then the fast rise time while a typical spike will fall about as quickly as it rises. Accordingly, such `slower` falloff noise spikes will be interpreted in the prior art as either noise, on the one hand(for example, in Ekwall's system) or as physiologic sensing on the other(unless it is simply filtered out), when in fact, significant and important information can be determined from monitoring such NPS events. It will be readily apparent to the reader that information on immanent lead failure is critical to patient health. As yet no satisfactory indicator for imminent lead failure has been developed. Further, if the system implanted overlooks the NPS events, on the one hand, or over counts them as physiologic senses on the other, it is possible that sub-optimum pacing therapy will be the result.